Apparatus for the encapsulation of discrete particles



July 27, 1965 D. E. WURSTER ETAL 3,196,827

APPARATUS FOR THE ENCAPSULATION OF DISCRETE PARTICLES (\J =v INVENTORS,

DALE E. WURSTER JAMES A. LINDLOF FIG. 3

A TTORNE Yi y 1965 D. E. WURSTER ETAL 3,196,827

APPARATUS FOR THE ENCAPSULATION OF DISCRETE PARTICLES Filed Nov. 19,1962 2' Sheets-Sheet 2 IN VEN TOR-"5.

DALE E. WURSTER JAMES A. LINDLOF 12 @M ZMQ A TTORNE YGS United StatesPatent APPARATUS FOR THE ENAPSULATION 0F DldCRETE PARTICLES Dale E.Wurster, Madison, Wis, and James A. Lindlof, White Bear Lake, Minn,assignors to WISCOllSlIl Aiurnni Research Foundation, Madison, Wis, acorporation of Wisconsin Filed Nov. 19, 1962, Ser. No. 238,422 3 Claims.(Cl. 11824) This invention concerns a device or apparatus designed forthe encapusulation of discrete particles of macro-size, as opposed tomicro-particles or powders. Examples of such macro-particles arepharmaceutical tablets, sugar seeds, candy or confectionery items,fertilizer particles, chemical prills, grains and/or seeds, etc. Thisapparatus may be used for the encapsulation of particles or objects inthe size range of from about 30 mesh (US. Standard sieve size) to about3" in diameter.

An object of this invention is to provide for the appli cation ofcoating materials in such a manner as to provide complete encapsulationof the objects being coated. Another object of the invention is to applythe encapsulation coatings in such a manner as to provide for asubstantially uniform thickness of coating on all particles of a batchand on all surfaces of individual particles. A further object of thisinvention is to provide for the minimization or elimination ofattrition, deformation, and/ or breakage of the particles during theencapsulation process. A further object of this invention is to providefor the application of said encapsulation coatings in such a manner asto produce a surface finish which will be attractive to prospectivepurchasers or users of the encapsulated products. A still further objectis to prowide for the application of the encapsulation coatings in sucha manner as to prevent or minimize penetration of the coating solventinto the core of the substrate material.

Encapsulation coatings which may be applied by this device includeaqueous solutions such as sugar syrup, gelatine,carboxy-methyl-cellulose, methyl cellulose, and starch paste and organicsolutions such as celluloseacetate-phthalate, ethyl cellulose, zein,polyvinyl acetate, and other similar resins and waxes. Similarly,suspensions, slurries, melts, latexes, and emulsions may be applied ascoatings in this device.

Basically, this device consists of an apparatus in which the substrateparticles to be encapsulated pass rapidly upward through a zone offinely atomized droplets of coating while they are freely suspended in afast-moving air stream. The dried coated particles then move downward ina fluidized condition countercurrent to a flow of heated fluidizing gasand subsequently reenter the coating portion of the cycle. The cycletakes place within a single chamber which preferably is provided with apartition for separation of the upwardly and downwardly moving particlestreams. The chamber is preferably truncated near its bottom to providea cross-sectional area at the bottom of the chamber perhaps only 75 to90% of the cross-sectional area in the main body of the chamber. Thistruncation provides for an upward velocity of gas in the bottom of thechamber greater than the velocity in the main portion of the chamber andby arranging for the bottom of the descending fluidized bed of particlesto be thus constricted, preferably in the region remote from the coatingzone, the descending particles are given a further push in the directionof the coating zone. The taper of the truncated portion is normallyabout 30 from the vertical. The tapered portion may extend upward abovethe bottom of the coating chamber a distance of from about 2 to 12inches.

An important feature of this invention is the provision 3,196,327Patented July 27, 1965 of a screen or porous plate for the bottom of thechamber which has definite areas of varying mesh sizes or porosity. Thisscreen provides for a better control over the circulation of theparticles through its variation in mesh sizes than is provided in theapparatus suggested in copending application Serial No. 98,078, filedMarch 24, 1961, now abandoned. The screen will have a highly porous areafor determination of the fast-moving air stream in the coating zone andan area of relatively low porosity for determination of the fluidizedzone. Also, the screen will have a third area remote from the highlyporous area or coating zone with porosity characteristics greater thanthe low porosity area and which can be intermediate the other two toensure the continuance of all particles evenly in the cycle. Thesescreen portions will be referred to hereinafter as areas H, L and M. Theporosity of the medium area M will generally be about 1.5 to 3.5 timesthe porosity of the low porosity area L. The high porosity area H mayconveniently have a porosity of 3 to 4 times that of area L. A suitableratio of the openness of areas H to M to L is about 3.5/2/1; i.e. area Hmay be about 35% open, while area M is about 20% open and area L isabout 10% open. As noted above, the range of porosities of area H andarea M overlap and where desired the porosity of area M can be the sameas the porosity of area H.

High porosity area H is located under the coating portion of theapparatus. The medium porosity area is located adjacent a wall or thewalls of the coating chamber remote from area H. Area L, the area of lowporosity, is positioned intermediate the areas H and M, and like area Marea L is under the portion of the chamber which comprises the zone ofdescending fluidized particles.

7 Based on the area in the main body of the chamber, area H can be about515%, area M about 1040% and area L about 45-85%. In a preferredarrangement the cross-sectional area at the bottom of the chamber is 85%of the cross-sectional area in the main body of the chamber, i.e. thearea of the main body of the chamber is reduced 15 by the truncation(hereinafter referred to as area T). Area T is preferably about the sameas area M, except that area M should be at least about 10% of thecross-sectional area of the main body of the chamber. Thus, when thearea in the main body of the chamber is reduced 15% by the truncation,area M should be about 15% of the area in the main body of the chamberor about 13% of the area of the screen.

While the chamber can be truncated near its bottom, and in someinstances is preferred, as noted above, the chamber need not betruncated, in which case the crosssectional area throughout the chamberis the same as the cross-sectional area of the porous bottom. Where thechamber is truncated, the truncation should not exceed 25%, as thecross-sectional area of the porous bottom, through which the fiuidizinggas passes upwardly, should be at least of the cross-sectional area ofthe main body of the chamber.

The introduction of three gas streams with controlled velocities intothe bottom of the chamber via areas H, L and M, has proven to be adistinct improvement over the use of a single high velocity gas stream,and also to give better control of the cyclic flow of particles in thelarger chambers than obtainable with two gas streams one of highvelocity and one of low velocity. The provision of a third gas streamremote from the coating zone with a velocity greater than the lowvelocity fluidizing gas stream adjacent the coating zone has been foundto aid in the smooth flow of the particles, e.g. prevent stoppage ofparticles with the formation of unfluidized accumulations, and also toaid materially in the transportation of the particles from the bottom ofthe downwardly flowing fluidized bed where the particles movehorizontally above the porous bottom and pass under the partition to thecoating zone.

The partition in the coating chamber physically separates the coatingzone where the particles move upwardly, from the subsidence zone wherethe particles move downwardly in a fluidized bed countcrcurrent to theupwardly flowing gas streams from areas L and M. The partition isgenerally designed to enclose about 8 to 20% of the cross-sectional areaof the coating chamber. It can be so designed as to enclose a constantcross-sectional area, or such as to provide a decrease in area of about20% from the bottom to the top, or to provide a venturi-type internalspace with a reduction of area from the bottom flared inlet to a throatsection and then gradual expansion to an exit having area about the sameas the inlet area. The partition normally extends from a level about 2to 4- inches above the porous member to the level desired for the top ofthe fluidized bed. This level is generally about one to three times thediameter or smallest cross-sectional distance of the coating chamber.

Another partition may be provided for the removal of dust and chips fromthe substrate material. This partition would enclose a small portion,say about of the cross-sectional area of coating chamber and would haveopenings below the top level of the fluidized bed which are somewhatsmaller than the particle size of unbroken substrate particles.

The invention will be better understood by reference to the accompanyingdrawings, which are illustrative only, and in which FIGURE 1 is a frontelevation of apparatus according to this invention;

FIGURE 2 is a diagrammatic cross'sectional view of the apparatus, partlyalong the line 2-2 of FIGURE 1;

FIGURE 3 is a view of the screen as it would be seen along the line 3-3of FIGURE 1;

FIGURE 4 is a cross-sectional diagrammatic view of 'a modified form ofthe coating chamber; and

FIGURE 5 is a cross-sectional view along the line 55 of FIGURE 4.

Apparatus employing this invention would ordinarily include the blower11 which supplies fluidizing gas through duct 13 to heat exchanger 15.From there, the heated gas passes through duct 17 and flexibleconnection 20 to the plenum chamber 22. From plenum chamber 22 the gaspasses through distribution plate or screen 25 and into coating chamber27. From this chamber the exhaust gas passes out through the exhausthood to exhaust duct 33.

Inlet damper 36 is located at the inlet to blower 11, and outlet damper39 is located in duct 13, or duct 17. The combination of these twodampers or similar air control devices can control both the volume andpressure of the fluidizing gas delivered to coating chamber 27 With thisarrangement of gas control devices, blower 11 may be a centrifugal typeblower powered by a constant speed motor. Other types of blowers may bealternately used with a variable speed drive substituted for dampers tocontrol the volume and pressure of gas delivered. Gas volume and gaspressure sensing devices may also be incorporated into the apparatus toindicate the gas volume and pressure, or alternately to automaticallycontrol the gas flow and pressure.

Heat exchanger 15 is normally a steam coil type heat exchanger but couldalternately have electric heating elements or use other type of heatingmedium. Normally, the heat exchanger is automatically controlled todeliver gas or air heated to a preset temperature. This can beaccomplished by a modulating controller with a sensing element (notshown) located downstream from the heat exchanger in duct 17, flexibleconnection 20, or plenum chamber 22. Under certain circumstances, theapparatus can be operated without the use of a heating device for thefiuidizing gas, but such operation is generally at reduced efliciency.

As shown, the chamber 27 is provided with partition 42 which divides themain body of the chamber into zones C and S. Zone C is generally thesame cross-sectional area as is provided by the high porosity area H ofthe porous member 25. It should be noted that the partition 42 iselevated above the screen 25, allowing passage for particles underneaththe partition and over the partition. Atomizing nozzle 44 projectsthrough the screen 25 and is directed toward zone C. This atomizingnozzle 44 is supplied with coating from reservoir 48 by line 50,metering pump 53 and line 55. The use of a metering pump is notessential to the encapsulation process, but does provide better controland hence better quality. Atomization nozzle 44 may be an air atomizingnozzle, either internal mixing or external mixing, or an airless typespraying nozzle. In FIGURES 1, 2 and 3 this nozzle 44, and with it, zoneC, is located at the side of the coating chamber 27, while in FIGURES 4and 5 these are in the central portion of the chamber 27 FIGURE 3 showsthe hole 57 in the screen 25 through which the nozzle projects.

FIGURES 3 and 5 show in detail representative dcsigns for the porousdistribution plate or screen 25 to control the velocity or volume of gaspassing up into the various portions of coating chamber 27. Portion L ofplate 25 admits relatively low velocity gas to the fluidized bed zone Sof coating chamber 27. The openings in portion L provide for only about10% of open area. Portion H of plate 25 admits high velocity air tospray zone C of coating chamber 27. The openings in area H of plate 25provide for about 35% open area. Portion M provides excess fluidizinggas at the back or edges of coating chamber 27, by having about 20% openarea. This increased gas flow serves to make up for the velocity lossdue to the tapered section 60 of the chamber 27 and also serves to keepthe particles away from the walls of the coating chamber and in the maincirculation stream. Thus area H of the screen is located below coatingzone C of the coating chamber, while area L and area M are located belowsubsidence zone S of the coating chamber. The tapered portion 60, asshown in the drawings, may be located on the side of the coating chamberopposite the nozzle or may be arranged annularly around a centralnozzle.

Cleaning partition 63, as shown, is provided with the perforations 66,below the bed level of chamber S for reception of dust, etc. into theflue formed behind the chamber 27 which flue empties into the exhaustduct 33. This flue may be further provided with a clean-out port (notshown) at or near its lower extremity. In operation, the pressure belowthe top of the fluidized bed in zone S would force some of thefluidizing gas through the openings 66 into the flue space behindpartition 63 and would carry dust and undesirable fine particles withit. These would either settle in this flue space to be removed later, orbe carried up and out the exhaust.

Further provision can be made for temperature-sensing devices in theupper portion of coating chamber 27, or in exhaust hood 39, or exhaustduct 33, to indicate the temperature and/or degree of saturation of theexhaust gases. Said temperature or saturation level sensing devices canfurther be used to control either the rate of application of the coatingsolution or the temperature of the inlet gas, or both, to partially orfully automate the encapsulation process.

Provision may also be made for filters, either at the inlet to theblower or in the exhaust, or both. Provision may also be made for asuitable solvent recovery system in the exhaust gas stream, in whichcase the gas may or may not be recycled as a closed system.

The apparatus as shown is for batch operation. To facilitate unloadingafter encapsulation, plenum chamber 22 including plate 25 may be hingedto the back of coating chamber 27 and swung down. Exhaust chamber 33 maybe pivoted upwards or sidewards for filling the chamber.

In operation substrate particles are loaded into coating chamber 27 andthe apparatus is sealed. The blower forces heated air up through plate25, and a fiow of atomized liquid droplets is started from nozzle .4. Arapid flow of gas up through coating zone C aspirates particles from thebottom of subsidence zone S and carries them rapidly up through thespray of atomized droplets in coating zone C. Droplets impinge on andare also electrostatically attracted to the particles in coating zone C.When the coating fluid is of the proper surface tension, the depositeddroplets flow over the surface to form a very thin film which drieswithin a very short period using mainly the heat available on thesurface of the particles. The dried, surface cooled particles issue fromthe top of coating member C beyond the top of partition 42, and therapidly moving flow of gas diffuses outward. This outward diffusion ofthe gas flow lowers its velocity below the transport or support velocityfor the particles and allows them to fall or be eased down onto the topof the fluidized bed in subsidence zone S. The lower gas velocitythrough the fluidized bed is just below or at incipient fluidizationvelocity and allows the particles to move downward, en masse, in aweightless condition or very near to such a condition. While movingdownward in this bed, the particles are traveling countercurrent to theflow of heat gas and hence the surfaces of the particles become heated.Near the bottom of this bed, the tapered portion of the chamber forces amovement of the surface heated particles toward the coating zone, and aflow of aspirating gas through the plate carries these particles on intothe coating zone, completing the cycle.

Since this complete cycle is fairly fast, on the order of 10-30 seconds,the core or nucleus of the particles does not usually becomesubstantially heated. The surface of the particles becomes heatedsufiiciently to dry a very thin film, and the surface cools as the filmdries, and then becomes warmed again in the fluidized bed portion. Sincethe complete encapsulation process requires 10-60 minutes, it can beseen that the final film is composed of about 100 or more very thinlayers or increments.

Because the coating apparatus of this invention is so efiicient it maybe manufactured in a relatively small size, e.g. where the chamber is 6to 18 inches in diameter. Also, since the elegant coating produced isideally suited for the preparation of high-value products, the coatingchamber may be fabricated from a transparent synthetic resin material,for example, an acrylic resin such as Lucite. As shown in the drawings,this chamber may be supported on the table 69, which advantageously isalso provided with the guard members 70 and 75. The table 69 has acentral opening suitable for reception of the porous plate and theplenum chamber is affxed to the bottom of this table.

The apparatus may also be arranged as a plurality of communicatingcoating chambers and may also be adapted to continuous encapsulation byproviding a particle enery at one side of the chamber 27 and an exit atthe other side.

it is claimed:

1. An apparatus for the encapsulation of discrete particles comprising avertically disposed coating chamber, said chamber being divided bypartition means into a vertically disposed coating zone and a verticallydisposed subsidence zone, said zones being open communication at theirupper ends above said partition means and at their lower ends below saidpartition means, said chamber having porous closure means located belowsaid partition means whereby gas may be passed into said chamber, saidporous closure means being characterized by areas of differing porosity,the porosity of each of said areas being a measure of the amount of eacharea which is open to the passage of gas therethrough, said areas ofdiffering porosity comprising an area of high porosity located beneaththe said coating zone comprising from about 5% to about 15% of the crosssectional area of the coating chamber, area of medium porosity,comprising from about 10% to about 40% of the cross-sec tional area ofthe coating chamber, located beneath the said subsidence zone and beingseparated from the said area of high porosity by an area of low porositycomprising the remainder of the cross-sectional area of said porousclosure means, the amount of the high porosity area and the mediumporosity area open to the flow of gas therethrough being respectivelyfrom about 3 to 4 times and from about 1.5 to 3.5 times the amount ofthe low porosity area open to the flow of gas therethrough, the amountof the medium porosity area open to the flow of gas in no event beinggreater than the amount of the high porosity area open to the flow ofgas, and atomizing means located to direct an atomized encapsulatingmaterial into the said coating zone.

2. The apparatus of claim 1 in which the coating chamber is tapered atits lower end to provide a bottom crosssectional area about to of thecross-sectional area in the upper portion of the chamber.

3. The apparatus of claim 2 in which the area of medium porosity isabout equal to the difference in crosssectional area between the upperportion of the coating chamber and the bottom of the chamber.

References Cited by the Examiner UNTTED STATES PATENTS RICHARD D.NEVIUS, Primary Examiner.

1. AN APPARATUS FOR THE ENCAPSULATION OF DISCRETE PARTICLES COMPRISING AVERTICALLY DISPOSED COATING CHAMBER, SAID CHAMBER BEING DIVIDED BYPARTITION MEANS INTO A VERTICALLY DISPOSED COATING ZONE AND A VERTICALLYDISPOSED SUBSIDENCE ZONE, SAID ZONES BEING IN OPEN COMMUNICATION ATTHEIR UPPER ENDS ABOVE SAID PARTITION MEANS AND AT THEIR LOWER ENDSBELOW SAID PARTITION MEANS, SAID CHAMBER HAVING POROUS CLOSURE MEANSLOCATED BELOW SAID PARTITION MEANS WHEREBY GAS MAY BE PASSED INTO SAIDCHAMBER, SAID POROUS CLOSURE MEANS BEING CHARACTERIZED BY AREAS OFDIFFERING POROSITY, THE POROSITY OF EACH OF SAID AREAS BEING A MEASUREOF THE AMOUNT OF EACH AREA WHICH IS OPEN TO THE PASSAGE OF GASTHERETHROUGH, SAID AREAS OF DIFFERING POROSITY COMPRISING AN AREA OFHIGH POROSITY LOCATED BENEATH THE SAID COATING ZONE COMPRISING FROMABOUT 5% TO ABOUT 15% OF THE CROSS SECTIONAL AREA OF THE COATINGCHAMBER, AN AREA OF MEDIUM POROSITY, COMPRISING FROM ABOUT 10% TO ABOUT40% OF THE CROSS-SECTIONAL AREA OF THE COATING CHAMBER, LOCATED BENEATHTHE SAID SUBSIDENCE ZONE AND BEING SEPARATED FROM THE SAID AREA OF HIGHPOROSITY BY AN AREA OF LOW POROSITY COMPRISING THE REMAINDER OF THECROSS-SECTIONAL AREA OF SAID POROUS CLOSURE MEANS, THE AMOUNT OF THEHIGH POROSITY AREA AND THE MEDIUM POROSITY AREA OPEN TO THE FLOW OF GASTHERETHROUGH BEING RESPECTIVELY FROM ABOUT 3 TO 4 TIMES AND FROM ABOUT1.5 TO 3.5 TIMES THE AMOUNT OF THE LOW POROSITY AREA OPEN TO THE FLOW OFGAS THERETHROUGH, THE AMOUNT OF THE MEDIUM POROSITY AREA OPEN TO THEFLOW OF GAS IN NO EVENT BEING GREATER THAN THE AMOUNT OF THE HIGHPOROSITY AREA OPEN TO THE FLOW OF GAS, ANS ATOMIZING MEANS LOCATED TODIRECT AN ATOMIZED ENCAPSULATING MATERIAL INTO THE SAID COATING ZONE.